Method of laminate manufacture

ABSTRACT

A laminate such as a PCB material is formed by stacking resin impregnated prepreg sheets (10) between copper foils (12), applying heat and pressure around the periphery of the stack to form an edge-sealed unit, and subjecting a number of such units to a lay-up process in a separate clean area into which they are introduced via surface-cleaning means.

This invention relates to laminates and printed circuit boards and morespecifically to components employed in the manufacturing of suchlaminates.

BACKGROUND OF THE INVENTION

1) Field of the Invention

A printed circuit board laminate normally includes, as a component,several layers of a resin impregnated woven glass fiber cloth known asprepreg. On opposite sides of the prepreg are conductive copper foilsheets which are subsequently etched to produce the conductive tracks.

In the manufacturing process called lay-up, the requisite number ofsheets of copper foil and prepreg are assembled onto a pressplate. Astack of such lay-ups is often built up. This stack is often called abook. The entire book is heated under pressure to bond the prepreg tothe copper foil. After cooling, the sheets of laminate are separated andsubjected to further processing.

2) Description of the Prior Art

FIG. 1 exemplifies the prior art lay-up process, in which prepreg sheets1 and copper foils 2 are assembled between pressure plates 3.

One of the chief causes of contamination is the presence of resin dustand glass fibers from earlier processing of the prepreg. Because theprepreg is handled in the same area, particles of resin dust from theair land on the surface of the copper foil and press plates. Someparticles remain even after wiping and during the pressing theseparticles melt and adhere to the surfaces.

This causes resin spots on the surface of the foil which preventsatisfactory etching of the copper. These spots can also cause pits anddents in the surface of the copper during the lamination process. If thepress plates become contaminated with fused resin spots, they causedents on the surface of the laminate each time they are used. The resinspots are very difficult to remove from the press plates except bysevere abrasion which makes the press plates thinner and shortens theiruseful life.

One object of the present invention is to provide a method of lay-upwhich prevents any resin dust created in subsequent handling from beingcarried in the air onto the press plates. Another aspect is to allow thesurfaces of the copper foil, especially thinner foils such as 1/2 ozfoils, to be cleaned without wrinkling or damage prior to beingassembled in the book.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a metal-facedlaminate, comprising forming a stack of plastics sheets between a pairof sheets of metal foil, applying heat and pressure to the edges of thestack to fuse the edges so as to form a sealed edge around the stack toform a sealed unit, cleaning the surfaces of the sheets of metal foiland subsequently heating the sealed unit while applying pressure to itsmajor surfaces to fuse the stack into a laminate.

Preferably, the sealed edge is formed by applying sufficient heat in thearea of the edge to melt the plastics material locally, and sufficientpressure to the area of the edge to press the metal foil sheetssubstantially together.

Preferably, the plastics sheets are resin impregnated woven glass fiberprepregs and the metal foil is of copper.

Preferably, the two heating and pressing steps are carried out inphysically separated locations.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side view illustrating a prior art lay-upprocedure;

FIG. 2 is a schematic side view of an apparatus for forming a sealedunit;

FIGS. 3a and 3b are plan and side views, respectively, of the sealedunit formed by the apparatus of FIG. 2;

FIG. 4 is a schematic side view of a lay-up apparatus; and

FIG. 5 illustrates the lay-up step of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 2, a number of prepreg sheets 10 are arranged in astack 14 between copper foils 12 within a sealing apparatus. The sealingapparatus comprises a table 16 on which the stack 14 is placed, and aresilient compression pad 18 which is movable relative to the table 16to hold the stack 14 in a firmly compressed state. Sealing elements 20are then closed together to apply heat and pressure around the peripheryof the stack 14.

The prepreg sheets 10 and copper foils 12 are conventional materials ascustomarily used in PCB manufacturer and are therefore not described indetail.

The apparatus of FIG. 2 produces a sealed unit 24 as illustrated inFIGS. 3a and 3b, the unit being entirely enclosed by the copper foils 12which are sealed together in a peripheral seal area 22 by fused resinfrom the prepreg material. The bulk of the prepreg remains unaffected,and the sealed edge is subsequently discarded.

The sealing elements 20 may be any suitable elements which provide theheat and pressure required, typically metal plates heated by electricresistance elements.

Sealed units such as 24 are then processed in the apparatus of FIG. 4. Apress table 26 is housed within an enclosure 28, the interior being keptin an high state of cleanliness by hepa fitters indicated at 30. Thesealed units 24 are introduced into the enclosure 28 via a cleaningmachine 30, and caul plates or pressure plates 32 (see FIG. 5) areintroduced via a further cleaning machine 34.

The cleaning machines 30, 34 each comprise upper and lower cleaningrollers 36a, 36b coated with an adhesive material which removescontaminants from the surfaces of the articles passing through. Machinesof this nature are more fully described in EP-A-0 214 741.

In this way, as shown in FIG. 5, a stack of pressure plates 32 andsealed units 24 is built up on the press table to form a book. Meat andpressure are then applied by known means (not shown) to fuse each unitto form a circuit board suitable for etching.

After cooling, the book is removed, the pressure plates are re-used, andthe fused edges are cut from the boards to leave copper-faced laminateready for processing.

The preliminary formation of the sealed units has the advantage thatvery thin copper foils can be used yet have relatively stiff units whichcan be passed through the cleaning machine without wrinkling. Theinvention further allows the working faces of the foil and the pressureplates to be thoroughly cleaned immediately before lay-up, with thelay-up area entirely separate from the initial handling area. In thisway no resin dust is present in the lay-up area, or on the surfaces ofthe copper and the pressure plates.

The invention makes it possible to achieve faster press loading, morerapid lay-up, and reduced pits and dents, and to use stainless steelpressure plates.

We claim:
 1. A method of forming a metal-faced laminate, comprisingforming a stack of plastics sheets between a pair of sheets of metalfoil, applying heat and pressure to the edges of the stack to fuse theedges so as to form a sealed edge around the stack to form a sealedunit, cleaning the surfaces of the sheets of metal foil and subsequentlyheating the sealed unit while applying pressure to its major surfaces tofuse the stack into a laminate.
 2. A method according to claim 1, inwhich the sealed edge is formed by applying sufficient heat in the areaof the edge to melt the plastics material locally, and sufficientpressure to the area of the edge to press the metal foil sheetssubstantially together.
 3. A method according to claim 1, in which theplastics sheets are resin impregnated woven glass fiber prepregs, andthe metal foil is copper.
 4. A method according to claim 1, in which thetwo heating and pressing steps are carried out in physically separatedlocations.
 5. A method according to claim 4, in which a number of sealedunits are assembled interleaved with pressure plates to form an assemblywhich undergoes the second heating and pressing step.
 6. A methodaccording to claim 5, in which the assembly is formed within anenclosure, and the sealed units and pressure plates are introduced intothe enclosure via means for cleaning their major faces.